System and method for controlling a brake system in a vehicle

ABSTRACT

A method for controlling a brake system in a vehicle includes using a first brake pedal map when the vehicle has a first load; the first brake pedal map allows a first predetermined non-friction braking torque to be reached. The method further includes using a second brake pedal map allowing a second predetermined non-friction braking torque, lower than the first predetermined non-friction braking torque, to be reached. The second brake pedal map is used when the vehicle has a second load lower than the first load.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. provisional patentapplication No. 61/643,669 filed 7 May 2012, which is herebyincorporated herein by reference.

TECHNICAL FIELD

The present invention relates to a system and method for controlling abrake system in a vehicle.

BACKGROUND

Vehicles today are increasingly equipped with electric drive motors,which, in addition to propelling the vehicle, can capture braking energyto charge a battery. Depending on how the vehicle powertrain isconfigured, this process, known as “regenerative braking”, can occur atthe front axle, the rear axle, or both. There are other kinds ofnon-friction braking, for example, engine braking, that occur when thecompression of the engine provides a negative torque to the vehicledrive wheels. Where the engine is only connected to one axle, as in atwo-wheel-drive vehicle, or where the regenerative braking is onlyavailable at one axle, there may be competing interests between tryingto brake in such a way as to maximize non-friction braking, for example,to maximize energy capture in a regenerative brake system, and moreevenly distributing braking torque between the front and rear wheels toprovide better vehicle handling.

Adding complexity to the braking control system is consideration of thevehicle carrying load. This may be of particular concern with commercialvehicles where the difference between the loaded weight and unloadedweight is significant. If, for example, a brake system is configured tomaximize non-friction braking at the rear axle for the fully loadedvehicle, the brake system may over brake at the rear wheels when thevehicle is unloaded. In addition, if the brake pedal travel is mappedthe same for the loaded and unloaded conditions, the brake pedal may be“too sensitive” when the vehicle is in the unloaded condition—i.e., avery hard braking may occur for a very small amount of pedal travel.Conversely, if the brake system is configured to maximize non-frictionbraking at the rear axle for the unloaded vehicle, the brake system maynot utilize all of the available non-friction braking—e.g., it may notcapture all of the possible regenerative braking—when the vehicle isloaded. This may be due, in part, to the lack of sensitivity of thebrake pedal, which now may be depressed so far as to engage thevehicle's friction brakes before all of the available non-frictionbraking energy is utilized.

SUMMARY

At least some embodiments of the invention include a method forcontrolling a brake system in a vehicle. The method includes using afirst brake pedal map allowing a first predetermined non-frictionbraking torque to be reached when the vehicle has a first load, andusing a second brake pedal map allowing a second predeterminednon-friction braking torque lower than the first predeterminednon-friction braking torque to be reached when the vehicle has a secondload lower than the first load.

At least some embodiments of the invention include a method forcontrolling a brake system in a vehicle. The method includes braking thevehicle with at least some non-friction braking until a firstnon-friction braking torque is reached when the vehicle has a firstload, and braking the vehicle with at least some non-friction brakinguntil a second non-friction braking torque, lower than the firstnon-friction braking torque, is reached when the vehicle has a secondload lower than the first load.

At least some embodiments of the invention include a control system forcontrolling a brake system in a vehicle. The control system includes acontroller configured to brake the vehicle with at least somenon-friction braking until a first predetermined non-friction brakingtorque is reached when the vehicle has a first load, and to brake thevehicle until a second predetermined non-friction braking torque, lowerthan the first predetermined non-friction braking torque, is reachedwhen the vehicle has a second load lower than the first load.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a simplified schematic diagram of a vehicle having acontrol system in accordance with embodiments of the present invention;

FIG. 2 shows a brake distribution chart for a vehicle in a fully loadedcondition;

FIG. 3 shows a brake distribution chart for the vehicle in an unloadedcondition using the same braking torque control as shown in FIG. 2;

FIG. 4 shows a brake distribution chart for the vehicle in the unloadedcondition using a different braking torque control;

FIG. 5 shows a brake distribution chart for the vehicle in the loadedcondition using the same braking torque control as shown in FIG. 4;

FIG. 6 shows a flowchart illustrating a method in accordance withembodiments of the present invention;

FIG. 7 shows a chart illustrating the relationship between vehiclebraking torque and pedal input in accordance with embodiments of thepresent invention; and

FIG. 8 shows additional details of the brake system shown in FIG. 1.

DETAILED DESCRIPTION

As required, detailed embodiments of the present invention are disclosedherein; however, it is to be understood that the disclosed embodimentsare merely exemplary of the invention that may be embodied in variousand alternative forms. The figures are not necessarily to scale; somefeatures may be exaggerated or minimized to show details of particularcomponents. Therefore, specific structural and functional detailsdisclosed herein are not to be interpreted as limiting, but merely as arepresentative basis for teaching one skilled in the art to variouslyemploy the present invention.

FIG. 1 shows a simplified schematic diagram of a portion of a vehicle10. The vehicle 10 includes a friction brake system 12, controlled by abrake controller 14, and a non-friction, regenerative brake system 16,which is part of the vehicle powertrain. The regenerative brake system16 includes one or more electric machines, such as electric motors,which are operable to provide regenerative braking for the vehicle 10.The regenerative brake system 16 is controlled by a control system, orvehicle system controller (VSC) 18, which communicates with the brakecontroller 14, for example, through a controller area network (CAN). TheVSC 18 may include other controllers, such as a powertrain controlmodule (PCM), and in some embodiments, the brake controller 14 may beintegrated into the VSC 18. Thus, a control system in accordance withembodiments of the present invention may control various systems withinthe vehicle 10 by using a single controller, separate softwarecontrollers within a single hardware device, or a combination ofseparate software and hardware controllers.

The brake controller 14 receives vehicle operator inputs from a brakepedal 20, and the VSC 18 receives operator inputs from an acceleratorpedal 22. A brake sensor 24 (which can be more than one sensor), isconfigured to detect the position of the brake pedal 20, and send one ormore signals to the brake controller 14. Similarly, an accelerator pedalsensor 26 (which can also be more than one sensor), is configured todetect the position of the accelerator pedal 22, and send one or moresignals to the VSC 18. The VSC 18 and the brake controller 14 usevarious inputs, including the inputs from the sensors 24, 26, to decidehow to control the friction brake system 12 and the regenerative brakesystem 16. The friction brake system 12 operates to slow the speed ofrear vehicle wheels 28 and the front wheels (not shown) through theapplication of one or more friction elements in accordance with methodsknown in the art. The regenerative brake system 16 is also operable toreduce the speed of the rear vehicle wheels 28 by having at least oneelectric motor produce a negative torque which is transferred throughthe powertrain to the rear vehicle wheels 28.

The friction brake system 12 includes one or more sensors, representedin FIG. 1 by a single sensor 30. The sensor 30 is configured to sendsignals to the brake controller 14 related to various conditions withinthe friction brake system 12. For example, if the friction brake system12 should experience reduced braking capability, perhaps due to a lossof boost or the loss of a hydraulic circuit, the sensor 30 cancommunicate this condition to the brake controller 14, which in turncommunicates with the VSC 18. Similarly, the regenerative brake system16 has one or more sensors, represented in FIG. 1 by the sensor 32. Thesensor 32 may detect such conditions as motor speed, motor torque,power, etc. The sensor 32 communicates directly with the VSC 18, whichcan use these inputs in combination with the other inputs to control thebrake systems 12, 16.

The vehicle 10 also includes a body/chassis system 34. The body/chassissystem 34 includes structural elements of the vehicle 10, including suchthings as a vehicle suspension system. The vehicle wheels 28, shownseparately in FIG. 1, may be considered a part of the largerbody/chassis system 34. One or more sensors, shown in FIG. 1 as a singlesensor 36, are configured to detect various conditions of thebody/chassis system 34, and to communicate with the VSC 18. The sensor36 may detect such conditions as the deflection of, or the load on,various elements of the body/chassis system 34, as well as loaddistribution. Similarly, a sensor 38, which represents one or moresensors, is configured to detect conditions of the vehicle wheels 28,including the wheel speed. The sensor 38 is shown in FIG. 1communicating with the larger body/chassis system 34, which in turncommunicates with the VSC 18. Alternatively, the sensor 38 can bedirectly connected to the VSC 18.

In the embodiment shown in FIG. 1, the regenerative brake system 16 is arear-axle regenerative brake system, configured to capture brakingenergy from the rear wheels 28 only. Although embodiments of theinvention are described and illustrated in conjunction with a rear axle,regenerative brake system, other embodiments may include other types ofnon-friction braking, such as engine braking, and may also include frontaxle or four-wheel non-friction brake systems. With a regenerative brakesystem, it is often desirable to capture as much braking energy aspossible, while not allowing too great a difference in brakingdistribution between the front and rear brakes so as to affect vehiclehandling. Toward that end, a controller, such as the VSC 18, can beprogrammed to perform a number of steps in accordance with embodimentsof the present invention. Initially, a first predetermined, non-frictionbraking torque, which in this embodiment is a maximum desiredregenerative braking torque, for the vehicle 10 can be provided when thevehicle 10 is carrying a first load, which, for example, may be amaximum capacity load conveniently identified by the vehicle's “grossvehicle weight” (GVW). This value of the braking torque can be“provided” to the VSC 18 by direct programming, or information can beprovided to the VSC 18 and it can perform an internal calculation.

FIG. 2 shows a brake distribution chart 40 for the vehicle 10 at thevehicle's GVW, which, for example, may be 3000 kilograms (kg). The chart40 illustrates a rear deceleration for the vehicle 10 along the verticalaxis, and a front deceleration along the horizontal axis. The sum ofthese two decelerations is the total deceleration for the vehicle 10,which can be easily converted into a vehicle braking force or a vehiclebraking torque because there is a known relationship between each ofthese values. Therefore, providing a vehicle with a desired non-frictionbraking torque as discussed above can also be described and illustratedin terms of a rear deceleration as shown in the chart 40 in FIG. 2.

The chart 40 shows a number of curves, including an ideal brakedistribution curve 42. The ideal brake distribution curve 42 illustratesa theoretical line along which the front and rear brakes would lock-upsimultaneously. An equal pressure curve 44 is also illustrated in thechart 40, and represents a line along which equal pressure is applied toboth of the front and the rear brakes. The ideal brake distributioncurve 42 is not coincident with the equal pressure curve 44, because inpractice, a vehicle does not have an equal weight distribution betweenthe front and rear wheels. As shown in FIG. 2, the lines 42, 44 cross atpoint Z₁, which may be different for different vehicles and differentloading conditions of the same vehicle. A number of equal decelerationlines 46 are also illustrated in the chart 40, and indicate lines alongwhich the front and rear wheels of the vehicle 10 are deceleratingequally.

As described above, it may be desirable to optimize regenerativebraking—i.e., to capture as much energy as possible—while at the sametime ensuring that there is not an undesirable impact on vehiclehandling. For any given vehicle, and vehicle loading condition, the“optimum” amount of regenerative braking that can be captured from thefront, rear or both pairs of wheels of a vehicle can be estimated. Usingthe vehicle 10 at GVW as an example, a maximum desired regenerativebraking torque (in this example for the rear regenerative brake system)is shown in the chart 40 by the line 48, which generally illustrates therear regenerative braking balance for the vehicle 10 at GVW. In thechart 40, the maximum non-friction braking torque is shown as a reardeceleration of −2 meters per second squared (m/ŝ2). For the vehicle 10,this level of deceleration can be translated into a deceleration torqueof approximately 1700 Newton-meters (Nm). After reaching this maximumvalue, the line 48 slopes downward and toward the right of the chart 40,indicating a combination of front and rear braking, until the equalpressure curve 44 is reached.

The slope of the line 48 is generally less than the slope of the equaldeceleration lines 46, and is brought below the ideal brake distributioncurve 42 somewhere at or before the intersection point Z₁. The specificway in which the maximum rear braking torque (in this case −2 m/ŝ2) ischosen, and how the rest of the brake balance line (or curve) isdetermined, can be based on any number of factors a brake systemdesigner wishes to consider. In the examples of embodiments of thepresent invention described herein, the optimum non-friction brakingtorque—which in this case coincides with the optimum regenerativebraking torque—is chosen to provide a “maximum desired” amount ofregenerative braking while still providing a required level of vehiclehandling. Although the first part of the curve 48 is vertical,indicating exclusive use of the regenerative (rear) brakes until adeceleration of 2 m/ŝ2 is reached, the initial deceleration may alsoinclude some front braking, as indicated by the line 48′, whichintersects the sloping part of the line 48 and follows its path fromthere. As braking occurs along the line 48′, and along the slopedportion of line 48, it may be a combination of friction and non-frictionbraking, or, in the case where non-friction braking is available at bothaxles, it may be exclusively non-friction braking even though both setsof wheels are braking.

As discussed above, embodiments of a method of the present invention maybe executed, for example, by the VSC 18. Using information, for example,from the chart 40 in FIG. 2, the method may include braking the vehicle10 with at least some non-friction braking (such as regenerativebraking) until the first predetermined non-friction braking torque (inthis case −2 m/ŝ2) is reached. This does not mean that non-frictionbraking ceases once the first non-friction braking torque is reached;rather, it means that non-friction braking is controlled to not exceedthis level (as discussed above, the sloping portion of the line 48 mayinclude non-friction braking, but along this portion of the line 48, theamount of rear axle braking is being reduced). The first predeterminednon-friction braking torque is based on the vehicle 10 at a first load,which as described above, is its GVW. One of the reasons that the chosennon-friction braking torque is load dependent, is because brakingconditions change with a vehicle when it is loaded versus when it isunloaded.

This is illustrated in FIG. 3 where a braking distribution chart 50 forthe vehicle 10 is shown when it has a second load lower than the firstload; in this case the vehicle 10 is unloaded—i.e., it is not carryingany payload. Thus, in FIG. 3, the weight of the vehicle 10 is equal toits “curb weight”. Although the fully loaded GVW weight and the unloadedcurb weight are used in the examples described and illustrated herein,it is understood that embodiments of the invention may be applied to anyor all of the various loading conditions that may exist between thesetwo extremes. In the chart 50, the equal deceleration lines 46 and equalpressure curve 44 are the same as in FIG. 2, while the brake balancecurve 52 and ideal brake distribution curve 54 are different from theircounterparts 48, 42 shown in FIG. 2.

If the same level of braking torque is applied to the vehicle 10 at itscurb weight as was applied at GVW (1700 Nm, see above), the result is agreater rear deceleration as shown by the brake balance curve 52 in thechart 50 in FIG. 3. In this example, the rear deceleration has increasedfrom −2 m/ŝ2 to −-2.8 m/ŝ2, as indicated by the label “Overbraking RearAxle”. As discussed above, this level of rear braking may beundesirable; therefore, embodiments of the present invention may utilizedifferent non-friction (in this embodiment, rear) braking torques fordifferent loading conditions of the same vehicle. This is illustrated inFIG. 4, which shows a braking distribution chart 56 for the vehicle 10at the second loading condition, which is its curb weight. In thisexample, a different optimum rear braking torque has been chosen so asto provide the desired vehicle handling throughout the braking event;this is indicated by the brake balance curve 58.

As shown in the chart 56 in FIG. 4, the maximum rear deceleration is−1.3 m/ŝ2, which translates into a rear braking torque of approximately900 Nm. Therefore, a system and/or method in accordance with embodimentsof the present invention may provide a second predetermined non-frictionbraking torque lower than the first predetermined non-friction brakingtorque when the vehicle has a second load lower than the first load. Thevehicle 10 is then braked exclusively with the rear brakes 28 usingregenerative braking until the first predetermined non-friction brakingtorque of 1700 Nm is reached when the vehicle 10 is at GVW; however,when the vehicle 10 is at its curb weight, it is braked exclusively withthe rear brakes 28 using regenerative braking only until a secondpredetermined non-friction braking torque of 900 Nm is reached. Asdescribed above, the first and second predetermined non-fiction brakingtorques of 1700 Nm and 900 Nm represent first and second desiredregenerative braking torques for the vehicle 10 for the two differentloading conditions, each of which may be maximum desired values for therespective loading conditions. Although the examples above rely onexclusive use of the rear brakes until the desired non-friction brakingtorque levels are reached, different embodiments may use a combinationof front and rear brakes, such as described above in conjunction withthe braking curve 48′ shown in FIG. 2. Moreover, some non-frictionbraking may still occur along the sloping portion of the line 58, butnon-friction braking is controlled to not exceed the secondpredetermined non-friction braking torque.

As described above, embodiments of the present invention provide twodifferent maximum desired regenerative braking torques for two differentloading conditions of a vehicle, such as the vehicle 10. Using themaximum desired regenerative (in this case, rear) braking torque from afully loaded vehicle for the same vehicle at a lower load resulted inthe undesirable effect of overbraking the rear axle, which wasillustrated and described in conjunction with FIG. 3. It is similarlyundesirable to use the maximum desired regenerative braking torqueprovided for the unloaded condition—such as illustrated and described inFIG. 4—when the vehicle has a higher payload. This is illustrated inFIG. 5, where a region of lost regenerative braking energy is labeled“Lost Regen at GVW”. This results from abandoning regenerative brakingtoo soon—i.e., at a braking torque level that is below the maximumdesired level.

FIG. 6 shows a flowchart 60 summarizing a method and system inaccordance with embodiments of the present invention. At step 62, theprocess is started, and at step 64 a determination is made as to loadestimation for a vehicle, such as the vehicle 10. This load estimationcomes from inputs 66, for example, to the VSC 18, that may provideinformation on the load level and “load quality”. Information such asthis can come from, for example, a sensor or sensors such as the sensor36 shown in FIG. 1. A sensor that detects deflection levels of asuspension system is one example of a load detection sensor. The “loadquality” factor may be provided to give an indication of the accuracy ofthe sensor itself, or the accuracy of the particular measurement as itrelates to the vehicle payload—i.e., a weight sensor may provide ahigher quality measurement than a deflection sensor, which must be usedin a calculation to estimate the actual load.

Next, at step 68, a front-to-back distribution of vehicle load isdetermined based on inputs 70 providing a front-to-back loaddistribution detection and distribution quality. When a vehicle load isdistributed toward a front of the vehicle, which may be defined, forexample, as in front of the rear axle, or in front of a center ofgravity for the vehicle, it may not be possible to provide a desiredlevel of braking torque at the rear axle without having an impact onvehicle handling. Therefore, a system and method in accordance withembodiments of the present invention may choose an initial value for thefirst predetermined non-friction braking torque, such as illustrated anddescribed in FIG. 2 for the rear wheels, and may also choose an initialvalue for the second predetermined non-friction braking torque, such asillustrated and described in FIG. 4 (also for the rear wheels). Then, ifit is determined that the first or second loads are distributed toward afront of the vehicle, the first and second predetermined non-frictionbraking torques can be modified such that they are reduced to a somewhatlower level to account for the low distribution. Although the “secondload” illustrated and described above was considered a zero payload forthe vehicle 10, the center of gravity of the vehicle at curb weight maybe distributed toward a front of the vehicle, and even in an unloadedcondition, the load distribution may be a factor to consider.

At step 72, a determination of brake torque level is made based on brakepedal input, load, and load distribution. To make such a determination,a controller, such as the VSC 18, may receive a brake pedal inputindicated at 74, for example, from a brake pedal 20 and sensor 24 shownin FIG. 1. Other inputs are shown at 76 in FIG. 6 which are brake levelversus pedal input maps, described below in conjunction with FIG. 7. Theprocess shown in FIG. 6 is ended at 78.

FIG. 7 shows a graph 80, which shows brake level versus pedal input mapsfor a vehicle, such as the vehicle 10 under two different loadingconditions: a GVW loading with good distribution and a minimum loadingwith poor distribution. The first condition is illustrated by a firstbrake pedal map, shown as a first curve 82, while the second conditionis illustrated by a second brake pedal map, shown as a second curve 84.Although the vertical axis of the graph 80 is labeled as “vehiclebraking torque” it is understood that it could be labeled in terms ofbraking force or deceleration as described above. Along the horizontalaxis is “pedal input” which relates to the travel of a brake pedal, suchas the brake pedal 20 shown in FIG. 1; thus, points on the curves 82, 84at different positions along the pedal input axis represent differentbrake pedal positions.

The curves 82, 84 respectively represent the vehicle braking torque asfirst and second functions of the brake pedal position for the vehiclehaving first and second loads, such as described above. These functionscan be programmed into the VSC 18 as formulas, if they can be definedthat way, or as data tables that can be accessed, with certain valuesoutput based on certain inputs received. Thus, the VSC 18 may outputvehicle braking torque as functions of brake pedal position. Thesefunctions, and the curves that represent them, such as the curves 82,84, can be chosen by a brake system specialist so that different valuesof vehicle braking torque are achieved for different pedal inputs. Acurve representing a maximum load with a poor distribution might appearbelow the curve 82, while a curve representing a minimum load with agood distribution might appear above the curve 84, but below the second,lower maximum load curve, and these would represent alternative pedalmaps. As shown in FIG. 7, the second brake pedal map 84 corresponds to alower set of vehicle braking torques than the first brake pedal map 82over a range of brake pedal positions or pedal inputs.

As discussed above in conjunction with FIG. 2, the total vehicle brakingtorque can be obtained by adding the front and rear decelerations andconverting this sum to a braking torque, which is shown on the verticalaxis in the graph 80 in FIG. 7. Because the level of pedal input for aparticular vehicle braking torque or deceleration has an impact ondriver expectations, it may be desirable to control the level of pedalinput for any given vehicle braking torque. In addition, as discussedbelow in conjunction with FIG. 8, some vehicles may engage frictionbrakes when the pedal has traveled a certain distance or angle;therefore, controlling pedal input versus vehicle braking torque may beimportant to ensure that the maximum desired non-friction braking isachieved before the friction brake system engages or becomes theexclusive braking mechanism.

Thus, with regard to the examples described above, a controller, such asthe VSC 18, may provide the first predetermined non-friction brakingtorque, or first maximum desired regenerative braking torque, at a firstposition of a brake pedal, such as the brake pedal 20 shown in FIG. 1.The first maximum desired regenerative braking torque is indicated bypoint (a) on the curve 82, which corresponds to a pedal position of(d₁). This point may be chosen, for example, to ensure that the firstmaximum desired regenerative braking torque is reached before thefriction brakes are engaged. This is illustrated in FIG. 8, which showsa gap 86 between the brake pedal 20 and the point of engagement of thefriction brake system 12. Although FIG. 8 is a simplified schematicdrawing, it does illustrate the general relationship between brake pedaltravel and friction brake system engagement that exists in somevehicles. Therefore, one factor to consider in providing a brake pedaltravel of a certain distance for a certain vehicle loading condition isthe distance allowed before the friction brake system is engaged.

The VSC 18 may also provide the second maximum desired regenerativebraking torque, at a second position of the brake pedal. If it isdesired to have the sensitivity of the brake pedal at the same level orperhaps slightly more sensitive when the vehicle is in the unloadedcondition, the second position of the brake pedal may be equal to orless than the first position of the brake pedal. This is illustrated inthe graph 80 where the second maximum desired regenerative brakingtorque is shown as point (b₁) on the curve 84, which also corresponds tothe first pedal position (d₁). If, however, a somewhat more sensitivebrake pedal is desired for lower loading conditions, the curve 84 couldbe adjusted to include the point (b₀) such that the corresponding pedalposition was (d₀). If the first pedal position (d₁) is chosen to be thesame or nearly equal to the width of the gap 86, then it may not bepossible to make the brake pedal less sensitive for a lower loadingcondition, otherwise, the brake pedal may engage the friction brakesystem before the maximum desired regenerative braking is achieved.

As shown in FIG. 7, the curves 82, 84 are not parallel over most oftheir range. They begin to approach parallelism at higher pedal inputpositions. This is because when the brake pedal is at or near abottoming out position, front and rear wheels are braking simultaneouslyand the difference between braking during a loaded and unloadedcondition is negligible. Although the curves 82, 84 could be made to beparallel over their entire range, having them nonparallel as shown inFIG. 7 provides some advantages. For example, at lower levels of pedaltravel, the curve 82 rises more steeply than the curve 84. This meansthat for the same change in pedal travel, a greater vehicle brakingtorque is applied when the brake system is controlled in accordance withcurve 82. This may result in similar deceleration regardless of theload; however, providing substantially similar curves at the higherlevel may provide tactile response (vehicle braking torque v. brakepedal travel) that may be desirable as it provides feedback to theoperator of the load that is being carried.

As described above, braking control according to the curve 82 is appliedwhen a vehicle has a large load with good distribution. Therefore, itmay be desirable to have a smaller amount of pedal travel provide agreater increase in braking torque, as this might be expected by avehicle operator when it is known that the vehicle has a large payload.The shapes of the curves 82, 84, and their underlying functions, can beadjusted to provide different vehicle braking torque outputs fordifferent pedal inputs as desired. In each case, however, the curves 82,84 provide brake pedal maps that allow the maximum desired regenerativebraking torque to be achieved before the friction brakes are engaged, orat least, before the friction brakes are exclusively engaged, whichcould cause less than the maximum desired regenerative braking to beachieved. Where the inputs are based on actual measurements and accurateinformation, the curves can be more aggressive; whereas, the use ofestimates and the absence of information may require a more conservativebraking control.

While exemplary embodiments are described above, it is not intended thatthese embodiments describe all possible forms of the invention. Rather,the words used in the specification are words of description rather thanlimitation, and it is understood that various changes may be madewithout departing from the spirit and scope of the invention.Additionally, the features of various implementing embodiments may becombined to form further embodiments of the invention.

What is claimed is:
 1. A method for controlling a brake system in avehicle comprising: using a first brake pedal map allowing a firstpredetermined non-friction braking torque to be reached when the vehiclehas a first load; and using a second brake pedal map allowing a secondpredetermined non-friction braking torque lower than the firstpredetermined non-friction braking torque to be reached when the vehiclehas a second load lower than the first load.
 2. The method of claim 1,wherein the first and second brake pedal maps are defined by vehiclebraking torque versus brake pedal input.
 3. The method of claim 2,wherein the second brake pedal map corresponds to a lower set of vehiclebraking torques than the first brake pedal map over a range of brakepedal inputs.
 4. The method of claim 1, wherein the vehicle includes arear-axle regenerative brake system, the first predeterminednon-friction braking torque corresponding to a maximum desiredregenerative braking torque when the vehicle has the first load, and thesecond predetermined non-friction braking torque corresponding to amaximum desired regenerative braking torque when the vehicle has thesecond load.
 5. The method of claim 1, further comprising providing thefirst predetermined non-friction braking torque at a first position of abrake pedal, and providing the second predetermined non-friction brakingtorque at a second position of the brake pedal equal to or less than thefirst position of the brake pedal.
 6. The method of claim 1, wherein thefirst brake pedal map is represented by a first curve, and the secondbrake pedal map is represented by a second curve, the first and secondcurves being nonparallel for at least a range of brake pedal positions.7. The method of claim 1, further comprising choosing an initial valuefor the first predetermined non-friction braking torque, and modifyingthe initial value of the first predetermined non-friction braking torquebased on a front-to-back distribution of the first load.
 8. The methodof claim 7, further comprising choosing an initial value for the secondpredetermined non-friction braking torque, and modifying the initialvalue of the second predetermined non-friction braking torque based on afront-to-back distribution of the second load.
 9. The method of claim 8,wherein the steps of modifying the initial values of the first andsecond predetermined non-friction braking torques include reducing thefirst and second predetermined non-friction braking torques when thefirst and second loads are distributed toward a front of the vehicle.10. A method for controlling a brake system in a vehicle comprising:braking the vehicle with at least some non-friction braking until afirst non-friction braking torque is reached when the vehicle has afirst load; and braking the vehicle with at least some non-frictionbraking until a second non-friction braking torque, lower than the firstnon-friction braking torque, is reached when the vehicle has a secondload lower than the first load.
 11. The method of claim 10, furthercomprising providing the first non-friction braking torque at a firstposition of a brake pedal, and providing the second non-friction brakingtorque at a second position of the brake pedal equal to or less than thefirst position of the brake pedal.
 12. The method of claim 10, furthercomprising defining vehicle braking torque as a first function of brakepedal position when the vehicle has the first load, and defining thevehicle braking torque as a second function of the brake pedal positiondifferent from the first function when the vehicle has the second load.13. The method of claim 12, wherein the first function is represented bya first curve, and the second function is represented by a second curve,the first and second curves being nonparallel for at least a range ofbrake pedal positions.
 14. The method of claim 10, wherein the vehicleincludes a rear-axle regenerative brake system, the first non-frictionbraking torque corresponding to a first desired regenerative brakingtorque when the vehicle has the first load, and the second non-frictionbraking torque corresponding to a second desired regenerative brakingtorque when the vehicle has the second load.
 15. The method of claim 14,further comprising: choosing respective initial values for the first andsecond desired regenerative braking torques; reducing the initial valueof the first desired regenerative braking torque from its initial valuewhen the first load is distributed toward a front of the vehicle; andreducing the initial value of the second desired regenerative brakingtorque from its initial value when the second load is distributed towarda front of the vehicle.
 16. A control system for controlling a brakesystem in a vehicle comprising: a controller configured to brake thevehicle with at least some non-friction braking until a firstpredetermined non-friction braking torque is reached when the vehiclehas a first load, and to brake the vehicle until a second predeterminednon-friction braking torque, lower than the first predeterminednon-friction braking torque, is reached when the vehicle has a secondload lower than the first load.
 17. The control system of claim 16,wherein the vehicle includes a rear-axle regenerative brake system, thefirst predetermined non-friction braking torque corresponding to amaximum desired regenerative braking torque when the vehicle has thefirst load, and the second predetermined non-friction braking torquecorresponding to a maximum desired regenerative braking torque when thevehicle has the second load.
 18. The control system of claim 17, whereinthe controller is further configured to receive inputs related to afront-to-back distribution of vehicle load, and to reduce the first andsecond predetermined non-friction braking torques when the first andsecond loads are distributed toward a front of the vehicle.
 19. Thecontrol system of claim 16, wherein the control system is furtherconfigured to receive inputs corresponding to brake pedal position, andto control the brake system to: reach the first predeterminednon-friction braking torque at a first position of the brake pedal, andreach the second predetermined non-friction braking torque at a secondposition of the brake pedal equal to or less than the first position ofthe brake pedal.
 20. The control system of claim 16, wherein the controlsystem is further configured to output vehicle braking torque: as afirst function of brake pedal position when the vehicle has the firstload, and as a second function of brake pedal position when the vehiclehas the second load, the first function yielding higher vehicle brakingtorques than the second function over a range of pedal positions.